This Phase 2, double-blind, placebo-controlled, randomized, multi-dose, multicenter study of MDX 1100 (anti CXCL10 human monoclonal antibody) in combination with MTX in subjects with active RA. All subjects will continue to receive stable doses of MTX (10 to 25 mg weekly) during the study. Eligible subjects (n=70) will be randomized to receive either placebo (n=35) or MDX-1100 (n=35) at 10 mg/kg intravenously, every other week for a total of 6 doses. Concomitant treatment with stable doses of prednisolone (≤ 10 mg/d, or equivalent) and non-steroidal anti-inflammatory drugs (NSAIDS) and analgesic drugs will be permitted during the study, however, the dose should not be changed until after the Day 85 assessment has been completed unless rescue therapy is required for significant worsening symptoms prior to Day 85. After Day 85, subjects will be followed until Day 141 only for safety and pharmacokinetics, and changes to baseline medications or addition of new medication will be permitted at the Investigator's discretion. Non-steroid anti-inflammatory drugs (NSAIDS) or analgesics should not be administered prior to disease activity assessments on study visit days.